The methyl ester of vinyl carbamic acid has been known for at least 40 years as a monomer which can be readily polymerized. Example 4 of U.S. Pat. No. 2,592,254, Dickey (1952) describes copolymerizing vinyl acetate and the methyl ester of vinyl carbamic acid to form a solid molding resin. The vinyl carbamic acid ester is said to have been prepared by reacting methyl urethane and acetylene. In general the patent discloses preparing amides or esters of unsaturated carbamic acids by heating urea or urethane with acetylene in the presence of a catalyst, such as a mixture of potassium hydroxide and zinc oxide. The process requires heating acetylene under pressure of at least 25 atmospheres using temperatures in the range of 150.degree. to 250.degree. C.
The polymerizability of N-vinyl-O-methyl carbamate is also disclosed by Bull. Soc. Chem. Belg. 66, pages 229-43 (1957) wherein it is stated that this monomer can be polymerized in solution in the presence of diazo-bis-isobutyronitrile. The synthesis of N-vinyl carbamates such as methyl and ethyl vinyl carbamates is by reaction of vinyl isocyanate with methyl or ethyl alcohol in the absence of a solvent. The melting point for N-vinyl-O-methyl carbamate which is soluble in methanol is given as 48.degree.-9.degree. C.
Another route to vinyl carbamates is disclosed by U.S. Pat. No. 3,019,231, Peppel, et al. (1962) which describes the vinylation of urethanes by reaction with vinyl ethers catalytically. The preferred catalyst is a mercuric salt of a mineral acid, e.g. mercuric sulfate. In Example III, N-methyl ethyl urethane was vinylated by reaction with methyl vinyl ether, producing N-vinyl-N-methyl ethyl urethane, useful as a comonomer in resin formation.
U.S. Pat. No. 3,715,385, Wolgemuth, et al. (1973) describes still another method of obtaining N-vinyl carbamic acid esters by condensation-re-arrangement of acrylonitrile carbonate with methanol or ethanol. In Example I, methyl (N-vinyl carbamate) was obtained in 40% yield from carboethoxy acrylohydroxamate at 100.degree. C. using a sodium alkoxy tin catalyst. The carboethoxy acrylohydroxamate is said to decompose at reaction temperature to yield acrylonitrile carbonate.
More recently, U.S. Pat. No. 4,574,159, Hassdenteufel (1986) describes pyrolysis of N-.alpha.-methoxyethyl-O-methyl urethane to form N-vinyl-O-methyl urethane. Referring to the disclosure of Bull. Soc. Chem. Belg. (supra), it states that the process described therein cannot be readily practiced industrially because it relies upon thermal decomposition of explosive acryloyl azide. The method of the invention described in this patent obtains the pyrolysis starting material by electrochemical oxidation of N-ethyl-O-methyl urethane. The pyrolysis splits off methanol to form the vinyl group. Recovery of the N-vinyl-O-methyl urethane is by cooling the pyrolyzed mixture to 20.degree. to -80.degree. C., thereby crystallizing out an almost pure product. The mother liquor can be used in another pyrolysis.
Another use for N-(1-alkenyl)-carbamates is disclosed by U.S. Pat. No. 4,459,236, Merger, et al. (1984) which indicates that such alkenyl carbamates can be converted to 1-alkenyl isocyanates at elevated temperatures. The process begins by reacting an aldehyde with a carbamate to provide the N-(1-alkenyl)-carbamate which is then split at elevated temperature. Methyl carbamate is listed as a suitable starting material. Acetaldehyde, although not a preferred starting material, is also listed among many suitable aldehydes. These materials can be reacted at 10.degree.-60.degree. C. in a solvent and in the presence of 0.001 to 0.05 equivalents of an acid such as HCl. Pyrolysis to the 1-alkenyl isocyanate is carried out in the vapor or liquid phase at 250.degree.-600.degree. C. This process appears to proceed contrary to the results described many years earlier in J. Org. Chem. 10, pages 483-497 (1945) which discusses condensation of carbonyl compounds by interacting acetaldehyde with ethyl carbamate in the presence of hydrochloric acid to prepare diethyl ethylidene dicarbamate. This study included a series of saturated aliphatic carbamates, from ethyl carbamate and higher aliphatic esters, interacted with a group of aliphatic carbonyl derivatives. Condensation was found to take place in the ratio of 2 moles of carbamate to 1 mole of carbonyl compound to form dialiphatic bis-carbamates with a small amount of concentrated hydrochloric acid serving as a catalyst to initiate the exothermic reaction. The product was recovered by crystallization. Methyl carbamate was not included in this study.
U.S. Pat. No. 4,572,804, Mullins (1986) says that .alpha.,.beta.-unsaturated monoiso-cyanates are the pyrolysis product obtained on heating .alpha.,.beta.-saturated geminal bis-carbamates. Pyrolysis conditions include temperatures from 100.degree. to 600.degree. C. for 0.5 to 5 seconds. The geminal bis-carbamates are preferably made by reaction between a carbamate and an .alpha., .beta.-unsaturated ether. In Example 2, vinyl isocyanate is made by pyrolysis of dimethyl ethylidene dicarbamate and distilling off the vinyl isocyanate. The undistilled material is said to be a mixture of carbamates including N-vinyl-O-methyl-carbamate.
U.S. Pat. No. 4,701,549, Mullins (1987) has the same disclosure, adding that this mixture which also contains methyl [(ethenylamino)carbonyl] carbamate can be polymerized to form a bonding resin. The pyrolysis of bis-compounds to form a polymerizable monomer is also described by U.S. Pat. No. 4,490,557, Dawson, et al. (1984) which discloses reaction of formamide and acetaldehyde in the presence of an acidic catalyst to form ethylidene bis formamide which can then be pyrolyzed to N-vinyl formamide, a monomer useful in preparing amine-functional polymers. Such polymers have attracted considerable interest because of their utility as flocculants and paper strength additives. The utility of N-vinyl formamide in this service stimulated my interest in the N-vinyl carbamates which can also be polymerized to a polymer hydrolyzable to contain amine functionality. Such a monomer offers an industrially attractive route to such polymers because urea and methanol can be easily reacted to form methyl carbonate which can then be reacted with acetaldehyde to form dimethyl ethylidene dicarbamate. An improved method for carrying out this latter process is the subject of my related co-pending application cross-referenced above. Pyrolysis of the bis-carbamate to vinyl carbamate rather than vinyl isocyanate presents a challenge which is addressed by the present invention.